Disk drives are commonly used in microprocessor based electronic devices, such as workstations, personal computers, laptops and other computer systems, to store and retrieve large amounts of data. A typical disk drive includes a plurality of magnetic disks that are rotated by a spindle motor and an actuator arm assembly that includes read/write heads mounted to flexure arms. An actuator motor (e.g., voice coil motor) can rotate the flexure arms and heads about a pivot bearing relative to the disks. The heads are configured to fly upon air bearings in very close proximity to the rotating disks.
The surface of each disk is divided into a series of data tracks which are spaced radially from one another across a band having an inner diameter and an outer diameter. The data tracks extend circumferentially around the disks and store data in the form of magnetic flux transitions on the disk surfaces. Each data track is divided into a number of data sectors that store fixed sized blocks of user data. Embedded among the data sectors on each track are servo fields that define servo information that enables the disk drive to control the radial position of the heads relative to tracks on the disks and to determine the circumferential location of the heads.
The servo fields may be written to the disks during the manufacture of a disk drive using a highly precise servo track writer, which utilizes the heads of the disk drive to write the servo fields. Because the servo fields are used to define the radial locations of tracks and the location of data along a track, it is important to precisely control the locations on the disk surfaces at which the servo fields are written. Thus, a typical servo track writer includes a precise clock signal generator and an additional recording element that is used to write a reference clock pattern on a disk surface responsive to the clock signal. The reference clock pattern is then read back from the disk surface by the additional recording element to generate a disk reference clock signal synchronized to the rotation of the disks, which is used to determine where to write the servo fields through the read/write heads onto the disks.
Disk drives have been developed that self-servo write the servo patterns without a servo writer. For example, an incremental two-pass self-servo write process begins with a first pass that writes reference servo patterns at a position determined by a crash-stop (the mechanical limit of the head's movement) and then servos on the reference servo patterns and writes the next set of reference servo patterns. The first pass repeats as the head moves radially across the disk, with each step servoing on the previously written reference servo patterns to write the next reference servo patterns at the next radial position. During the first pass, the servo loop has no absolute reference to ensure placement of the reference servo patterns at the appropriate radius. After the first pass finishes the complete stroke, a second pass writes the final servo patterns using the reference servo patterns to find the appropriate positions. However, the second pass substantially increases the self-servo writing time.
In some disk drives, spiral patterns are written onto the disk, and the disk drive self-writes servo patterns on the disk using the spiral patterns as a reference for servoing the head. For example, 100-200 spirals may be written onto the surface of the disk. The spirals generally form an outwardly-expanding pattern from an inner diameter of the disk to an outer diameter of the disk. However, there may be small variations in the circumferential spacing of the spirals. Some aspects of these variations are predictable, however, because the variations repeat with each revolution of the disk. Such variations may cause the servo patterns to be written at incorrect locations on the disk. Accordingly, when self-writing the servo patterns on the disk, it is desirable to predict and account for these variations.